Mine wastewater, for example, contains pyrite (FeS2), which oxidizes to produce SO42−. In order to neutralize the mine wastewater, Ca(OH)2, which is low-cost, is used. Therefore, the mine wastewater contains Ca2+ and SO42− in abundance.
It is known that saline water, sewage and industrial waste water also contain Ca2+ and SO42− in abundance. In a cooling tower, heat exchange is performed between cooling water and high-temperature exhaust gas discharged from a boiler. A portion of the cooling water becomes steam due to this heat exchange, so the ions in the cooling water are concentrated. Accordingly, the cooling water discharged from the cooling tower (blowdown water) contains high concentrations of ions such as Ca2+ and SO42−.
The water containing large quantities of ions is typically released into the environment after being desalinated. Known examples of concentrating device for performing desalination treatment include a reverse osmosis membrane device, a nanofiltration membrane device, and an ion exchange membrane device.
However, in the desalination treatment using these devices, if cations (e.g. calcium ions (Ca2+)) and anions (e.g. sulfate ions, (SO42−)) at high concentrations are concentrated on a membrane upon recovering reclaimed water thereof, the concentrations may exceed the solubility limit of calcium sulfate (gypsum (CaSO4)), which is a poorly soluble mineral salt. This may become problematic, because deposition may be formed on the membrane surface as scales, causing the reduction in permeation flux (flux) of reclaimed water.
Therefore, monitoring methods for mineral salt crystalline formation have been proposed in the conventional art, such as a method in which a cell monitoring the reverse osmosis membrane in the reverse osmosis membrane device was used to monitor the reverse osmosis membrane and the formation of the mineral salt crystals was visually observed (Patent Document 1).